The invention relates to an endo knee prosthesis having a femur part and a tibia part which are adapted to be anchored, each by way of an intramedullary stem, in the femur and lower leg bone respectively and which are pivotable relatively to one another around a flexing-axis pin to an extent limited by abutments.
When it becomes necessary to replace a knee joint by a prosthesis as a result of damage due to accident or disease, the operating surgeon will endeavor to resect as little bone as possible and to select a prosthesis which is likely to be highly durable, so that the operation does not have to be repeated and the joint does not stiffen.
In the case of conventional total prostheses, it has been necessary to resect the widened heads from the femur and tibia, whereafter the intramedullary stems on the hinge-like prosthesis are introduced into the medullary cavity of the associated bones and anchored in place by bone cement. Considerable resection is necessary in this procedure and durability is limited since all that secures the prosthesis from turning in the bones is just the bone cement. Consequently, impact torque loadings of the joint occasionally cause the prosthesis to loosen and the loosening is not only painful but makes a further operation necessary.
To obviate these problems, German patent specification No. 2 114 287 proposes that the joint zone of the prosthesis be so narrow that the joint can be implanted inside the femoral head. This step really does obviate the difficulties mentioned by making total resection of the condyle unnecessary. Also, implanting the prosthesis in a recessed part of the condyle provides extra securing against turning. Unfortunately, with the known joint construction two disadvantages arise, for the reason that the prosthesis must be taken apart for implantation and, after the intramedullary stems have been positioned, the prosthesis must be assembled by way of the hinge or pivot pin. Consequently, a bore or recess extending medially through the condyle is necessary for such pin, with a resultant reduction of the unresected part of the condyle. Yet another disadvantage is that for anatomical reasons the bore is required in a part of the condyle where the lateral ligaments are attached, so that such ligaments have to be removed, even though undamaged, and therefore become unavailable to damp forces and to provide additional guiding of the joint.